1. Field of the Invention
The present invention relates to a method of heating and bending a glass sheet, and more particularly to such a method in which the vertical distance between the glass sheet and a furnace heat source is varied. The invention also relates to apparatus for heating and bending a glass sheet in which this distance is varied. Bent glass sheets thus produced may be used in the manufacture of glazings for vehicles.
2. Description of the Related Art
It has long been recognised that it is desirable to be able to vary the vertical distance between a glass sheet and a heat source by which the sheet is heated. This technique provides a means of control of the heating effect which the heat source has on the glass sheet, and is thus a further means additional to, for example, control of heating effect by regulating the heat output of the heat source, or directing the heat by means of reflectors, shields or baffles. Control of the heating effect produced in the glass sheet is needed in many situations, for instance in glass bending to be able to produce bent glass sheets of various different shapes each requiring different degrees of heating in certain areas of the sheet to obtain the desired different shape on bending. Careful control of heating is especially important when producing bent glass sheets of complex curvature, i.e. having curvature in two directions at right angles to each other.
The need to vary the vertical distance between glass sheet and heat source occurs frequently in the bending of glass sheets for use in the manufacture of glazings for vehicles. Such sheets are commonly heated to a softened condition for bending while supported on a bending mould in a furnace. In this situation, there may be a need to vary the vertical distance between sheet and heat source during heating of the sheet. Generally, this arises because it is desired to heat one portion of the glass sheet to a higher temperature than another portion, i.e. differentially heat the glass sheet; for example, create a centre-edge temperature profile across the sheet to assist in obtaining a particular curvature, or locally heat the sheet to a higher temperature to obtain a sharper bend along a particular axis, e.g. by using an auxiliary heat source such as a crease heater. In these situations, one wishes to reduce the vertical distance between the glass sheet and the heat source at a certain stage during the heating of the sheet, and it is known to raise or lower heat sources to achieve this. However, when it is desired to vary the vertical distance between the sheet and a large number of heat sources, it is preferable from an engineering aspect to raise the sheet towards the heat sources.
A known apparatus in which the vertical distance between a glass sheet and a heat source is varied by raising the sheet towards the heat source is described in EP 476 693, corresponding to U.S. Pat. No. 5,194,083. In this apparatus, a glass sheet is placed on a bending mould on a carriage and transported to a position below a furnace. The carriage is then lifted into the furnace by means of a hoist. Additionally, the distance between the glass sheet and heating elements in the top section of the furnace may be varied by raising or lowering a movable floor of the carriage.
Clearly the rate of production attainable in such a furnace is very limited, as it can only accommodate one bending mould at a time. Furthermore, for more sophisticated control of the heating of a glass sheet, it is desirable to subject the sheet to different configurations of heating elements at different stages (as well as other control techniques e.g. different levels of heating power) during heating.
These requirements have been fulfilled by providing a glass bending apparatus comprising a furnace having a plurality of heating zones, wherein each zone can accommodate one or more bending moulds.
Such an apparatus is disclosed in U.S. Pat. No. 4,497,645. It comprises upper and lower transport runs, along which transfer cars are conveyed. The cars each have a single end wall which extends upwards towards the roof.
However, this document is concerned with improving the thermal efficiency of the furnace, and not with varying the vertical distance between a glass sheet and the furnace heat sources. It does not assist in improving control of the heating effect produced in the sheet while it is in any one zone.